(1) Field of the Invention
This invention is generally directed to cooling systems for digital computer and other electronic systems and more particularly to apparatus for minimizing the high airborne and structureborne noise generated by such cooling systems while maintaining effective cooling of electronic components and systems.
(2) Description of the Prior Art
Personal computers, workstations and other electronic systems typically utilize internal power supplies to deliver power to components and peripherals disposed within the systems. In a computer environment, peripheral devices are added using expansion slots. As more peripheral devices are added, the internal power supply must deliver more current with a concomitant increase in heat being dissipated. Consequently the cooling requirements increase.
Forced convection cooling is the most widely used method for dissipating heat in such electronic systems. With forced convection cooling, fans direct air across the surface of electronic components typically inside a chassis. Some forced convection cooling system fans operate on alternating current (AC). Generally, rotational speed of AC fans cannot be easily changed. Consequently, AC fans rotate at a fixed speed that is determined by the maximum power requirement. If the power requirement reduces, the fan speed remains constant and therefore is excessive for these less demanding conditions. This excessive speed results in unnecessarily high levels of airborne noise caused by airflow through filters and over components, and structureborne noise caused by mechanical vibrations of the fans during operation. Excessive noise, as is known, can be disadvantageous in a number of applications.
Brushless direct current (DC) fans are becoming popular. They are reliable and are as capable as standard AC fans in cooling electronic components. The speed and resulting airflow of DC fans is proportional to the DC voltage applied. The ability to match fan speed to cooling requirements at any given time would lead to the possibility of reducing fan speed if the power supply were not operating at its maximum. Reducing fan speed by 20% produces a 5 db sound level decrease; a 40% reduction in fan speed produces a 10 db reduction in noise due to airflow and structureborne noise. There are several examples of control systems that control fan speed in response to operating parameters.
In U.S. Pat. No. 3,230,293 (1966) to Turgeon a current transformer monitors the load on a phase conductor in a multiphase electrical system. The output from the current transformer then serves to control an air foil fan that directs a cooling medium over the conductor. Thus in this reference, fan speed is dependent only upon the electrical load.
U.S. Pat. No. 5,436,827 (1995) to Gunn et al. discloses an alternative approach whereby the control boards in a power mixing circuit sense the speed of a fan and select one or the other of control boards for energizing the system. This provides parallel energizing circuits for the fan and prevents the inadvertent deenergization of the fan. The control in this patent is dependent upon fan speed as the loss of fan speed while powered by one control board will cause a shift to the other control board.
U.S. Pat. No. 5,484,012 (1996) to Hiratsuka discloses an electronic system with two fans. The electronic apparatus to be cooled is located in a housing and includes at least one heat source. The housing has an intake port and an exhaust port. The electronic apparatus comprises a cooling fan mounted in the exhaust port and an auxiliary cooling fan placed near the heat source. A first control portion controls fan speed in accordance with the temperature of intake air. A second control portion determines fan speed for the auxiliary cooling fan in a first mode when the fan speed of the cooling fan is less than a reference speed and in a second mode when the fan speed of the cooling fan is greater than the reference speed. Thus this patent discloses a control system in which fan speed is dependent upon the ambient temperature as represented by the temperature of the incoming air. This system does not provide for electrical load monitoring.
Still other patents disclose other systems for maintaining operating temperatures for electronic circuits. U.S. Pat. No. 4,685,303 (1987) to Branc et al. discloses a disk drive isolation system in which a thermo-electric heat pump maintains conditions within a disk drive in response to humidity. U.S. Pat. No. 5,121,291 (1992) to Cope et al. discloses a system in which internal temperature controls fan speed. U.S. Pat. No. 5,249,741 (1993) to Bistline describes a system that establishes fan speed based upon a particular configuration of equipment monitored during a computer operation start-up or "boot" operation. This system does not automatically adjust fan sped when an unknown device is installed.
The Hiratsuka patent in particular discloses a system that monitors fan speed and ambient temperature to control fan speed. The remaining references disclose for speed control based upon other parameters. However, none of these patents disclose fan speed control that responds to the precise measurement of the electrical load and the ambient temperature and that further controls fan speed in an attempt to reduce airflow and structureborne noise.